An electrically heated furnace is an enclosure in which electrical energy is used to produce high temperatures. An important type is the electric arc furnace in which scrap iron and steel is melted to produce ordinary steels, alloys and stainless steels. Typically the electric arc furnace has a cylindrical shell and dished bottom, both formed of steel plate and lined with refractory. The roof of the furnace is removable to allow charging of the furnace. After shredded scrap is introduced, the roof is swung back and lowered to close the furnace. Electrodes are lowered through holes in the roof until they contact the scrap charge. Electric current is then passed from one electrode down through an arc into the metal charge, then from the charge up through an arc to an adjacent electrode. The intense heat generated progressively melts the charge, and refining materials are added to provide a molten steel of desired composition. The furnace is tilted rearward to pour off slag and forward to pour out the metallurgically finished steel. If necessary, the furnace interior is repaired before the next heat is started.
The bottom or hearth of the furnace usually is a dished steel shape containing a lower layer and an upper layer of refractory. The lower layer, which can last for several years, is sometimes called the safety lining, and may be formed from several layers of brickwork. Over the safety lining, granular refractory material is rammed in until the desired bottom contour is formed. This upper layer lasts a number of heats and may be repaired as required.
While electrically heated furnaces can have just one electrode and use direct, single-, two- or three-phase current, furnaces for steelmaking usually have three electrodes and use three-phase current. Since the heating is localized around the electrodes, metal charge in the hearth remote from the electrodes melts later, and heat time is extended. The process consumes more energy and takes longer than would be otherwise necessary if the heating were more even throughout the furnace. Furthermore, stratification occurs in the composition as well as in the temperature of the furnace contents.
A method for increasing the heating and melting rate in an electric arc furnace is to agitate or stir the charge as it becomes molten. The agitation is often by discharging a gas, usually an inert gas, into the bottom or side of the furnace through a tuyere. Within the furnace, the tuyere typically comprises a refractory plug whose top surface is even with the furnace refractory surface. Problems, however, exist in practice.
Erosion or wear of the tuyere plug and the surrounding furnace refractory is accelerated by the agitation. As service time is accumulated and plug wear progresses, the gas discharge opening in the plug becomes increasingly susceptible to blockage. After pouring finished metal from the furnace, residual metal or slag may freeze over the opening. During patching of the working lining of the furnace between heats, patching material may inadvertently enter or deposit over the opening.
A further problem peculiar to the electric arc furnace arises from the high voltages and currents employed. Three phase voltage is supplied to the electrodes with the intent of maintaining the contents of the furnace at neutral. However, phase imbalance occurs at times causing an electric potential to develop in the furnace contents.
The furnace metal shell is grounded and electrically isolated by its refractory linings from the molten metal contents. However, in time, the refractory linings wear and crack, and their dielectric property degrades. Then if there is any phase imbalance, some electrical current may pass into the shell. With phase imbalance, electric current may flow through any conduction path in the tuyere plug into the tuyere piping and on into the tuyere external supports and gas supply piping. Arcing may also occur from any of these components to an adjacent surface at a lower potential, such as the furnance shell. The current flow in the tuyere plug may produce some electric heating in the plug thereby contributing to its deterioration.
The undesirability of electric current flow in tuyeres has been recognized and addressed by steelmakers. U.S. Pat. No. 4,735,400 to Tate et al describes a tuyere plug containing metal tubes for the passage and discharge of gas into the furnace interior. Within the plug, each tube is interrupted by a section of dielectric to prevent current flow in the tube. Such plug construction is costly and does not eliminate the possibility of electric current conduction through the plug refractory itself to the tuyere pipe. One of the most wear resistant of plug refractories, magnesia-graphite, has been determined to be conductive in the trials performed in evaluating the novel plug disclosed herein.
Accordingly, it is an object of this invention to increase the energy efficiency and shorten the time for heats in electrically heated furnaces.
It is an advantage of this invention that an economical, easily fabricatable, durable tuyere is provided with a long functional life and which does not arc in an electrically heated furnace.
It is a feature of this invention to have a dielectric interruption in the tuyere piping outside the furnace. It is another feature to have a tuyere plug with an integral sleeve, the plug and sleeve each being of selected wear resistance so as to maintain an even wear profile in the furnace interior surface.